1-oxo-1,2,3,4-tetrahydronaphthalene (also known as ".alpha.-tetralone") is generally known as a precursor of .alpha.-naphthol(A), which has been widely used as an intermediate of agricultural chemicals, pharmaceutical, dyes, and various precision-required chemicals. ##STR2##
It is well known that the catalytic liquid phase oxidation of tetralin (formula II), which has four comparatively weak C--H bonds, produces .alpha.-tetralyl hydroperoxide, .alpha.-tetralone, and .alpha.-tetralol. All of the above are oxidized only at the .alpha. - position, using molecular oxygen as an oxidant and a metal ion catalyst in the presence or absence of solvents. ##STR3##
.alpha.-tetralyl hydroperoxide is obtained in high selectivity from the oxidation of tetralin (Japanese Patent Laid Open No. 54-10248).
Subsequently, .alpha.-tetralone was prepared by decomposing .alpha.-tetralyl hydroperoxide under metal ion catalysts.
.alpha.-tetralone may also be prepared by the direct liquid phase oxidation of tetralin under catalysts such as the chromium salt, cobalt salt, pyridines, or amines (U.S. Pat. No. 3,404,183, Japanese Patent Laid Open No. 49-135958, 50-112347 and 51-48643).
However, a method for obtaining .alpha.-tetralone by the decomposition of peroxides has some recognized disadvantages. The reaction results in lower conversion and a slower reaction rate due to the handling requirements that thermally sensitive compounds demand and the need for low peroxide concentrations.
Furthermore, the direct oxidation method using cobalt salt and so on to obtain .alpha.-tetralone, also has some disadvantages. The decomposition rate for the produced peroxides was increased by a cobalt catalyst which caused the concentration of .alpha.-tetralone, a by-product in this reaction to increase, as a result, the selectivity for the desired ketone decreased.
In addition, a great deal of investment in operation and design of facilities in separating .alpha.-tetralone from .alpha.-tetralol become necessary because the temperature difference between .alpha.-tetralol and .alpha.-tetralone is only 2-3.degree. C. at the same vapor pressure.
Accordingly, an .alpha.-tetralone fraction, containing 5-60% of .alpha.-tetralol, is usually used as the raw material in the dehydrogenation reaction. In this case, the rate for dehydrogenation of .alpha.-tetralol is very rapid and readily converted to naphthalene through further dehydrogenation of the 1,2-dihydronaphthalene intermediate. However, naphthalene, a compound that has sublimation property and is used as an insecticide because of its toxicity, reduces the activity of the catalyst in the catalytic dehydrogenation reaction by being adsorbed at the surfaces and pores of the catalyst.
Especially, the use of the chromium salt instead of the cobalt salt markedly increases the ratio of .alpha.-tetralone to .alpha.- tetralol. However, the economic aspects of the process, such as safety, high cost of pyridine, slow reaction rate, and efficiency of recovery, renders the process unfavorable since a large amount of pyridine solvent is used.
In addition, liquid phase oxidation under a pyridinium salt catalyst, in the absence of solvent, exhibits high concentrations of .alpha.-tetralyl hydroperoxide. However, this process is also unfavorable since the same problems such as control and safety of the reaction, are seen.
A process for preparing .alpha.-tetralone by oxidation of tetralin, in high selectivity, under a catalyst system in which the soluble chromium or cobalt salt is dissolved in N,N-dialkyl acid amides such as dimethylformamide(DMF), dimethylacetamide(DMA), and diethylacetamide(DEA), is reported in Japanese Patent Laid Open No. 541-14950. In this process, a homogeneous solution is obtained by dissolving the metal salts in the solvent mentioned above. The peroxide is diluted sufficiently to a safe concentration range, and the product yield can be increased by promoting the decomposition reaction of peroxides.
In the above case, the catalytic soluble metal salts such as acetate, propionate, stearinate, and naphthenate salts are used as the soluble cobalt or chromium salt showing about 30% of conversion. The catalytic activity of these salts is thought to be a result of the formation of a chromium or cobalt complex with the N,N-dialkyl acid amide as a bidentate ligand. But, according to the above processes, the product yield was decreased because of a higher concentration of by-product.
The technology for using the 8th group noble metal complex catalyst with the biphyllic ligand is reported in U.S. Pat. No. 3,422,147. The use of the chromium oxide complex catalyst using lutidine as a ligand, complex catalyst between the chromium salt of organic acids and the pyridine derivatives are reported in Japanese Patent Laid Open No. 50-58044 and 50-112347 respectively. The use of chromium acetylacetonate is disclosed in Japanese Patent Laid Open No. 60-19744. However, all of the above processes suffer from low yields ranging from about 25.1 to about 33.4%.
An object of the present invention is to provide an improved and economic process for preparing .alpha.-tetralone through the liquid phase oxidation of tetralin in high selectivity and high yield.